Objective CVD diamond is a hard and brittle material of wide applications, but the disordered arrangement of coarse grains in polycrystalline CVD diamond leads to uneven surface. The commonly used methods for CVD diamond processing are ultra-precision grinding and chemical-mechanical polishing, but generally of low efficiency and tool life. The efficiency of laser ablation depends on the optical and thermal properties of the laser, which provides a highly directional and localised energy source for diamond processing. Therefore, laser proessing is suitable for CVD diamonds with high hardness and wear resistance. It is necessary to study the influence of laser parameters on the surface morphology and surface damage to achieve the parameters optimization for nanosecond laser polishing of CVD diamond.

Methods In this study, the surface generation process of laser ablated CVD diamond was firstly investigated by finite element simulation, and then the influence of laser processing parameter on the surface roughness and surface topographic characteristics of CVD diamond was investigated by single factor experiment. The surface roughness was measured using a 3D laser confocal microscope, and the surface topographic features of the workpieces were examined using a scanning electron microscope. The effects of laser incidence angle, laser power, laser scanning speed and scanning times on the surface roughness and surface topographic features of CVD diamond were achieved (Fig.6, Fig.11, Fig.14).

Results and Discussions The results of finite element simulation (Fig.4) show that the incidence angles of the laser affect the polished surface, and the greater laser incidence angle, the smaller removal depth of the material. The laser polishing of CVD diamond were carried out with different laser incidence angles and powers, and the experimental results (Fig.6) were consistent with FEM. When the laser power is higher, the surface roughness of the material decreases with increasing incident angle, while the effect of the laser incident angle on the surface roughness drops significantly when the laser power is lower. The laser polishing of CVD diamond under different laser scanning speeds (Fig.11) shows that the surface roughness of the material decreases firstly and then increases with the growth of laser scanning speed. When the scanning speed was lower, the a great number of larger-size graphite grains and grooves formed (Fig.13), and when the scanning speed was higher, the surface turned to be relatively flat but with a lot of small cracks among the graphite grains. Finally, different laser scanning times of CVD diamond (Fig.14) show that the surface roughness of the material firstly decreases and then increases with the increase of the number of laser scanning times. A growing number of laser scanning times leads to a number of cracks on the diamond surface, which worsens the surface roughness (Fig.16).

Conclusions In this study, the surface generation process of laser ablation of CVD diamond is investigated by finite element simulation and experiments, and the influence law of nanosecond laser processing parameters on the surface morphology and surface damage of CVD diamond is explored to achieve the optimization of nanosecond laser polishing parameters. The experimental results show that the increase of the laser incident angle can weaken the trapped light effect on the material surface, which can effectively improve the surface roughness of the material, and the greater laser incident inclination angle, the lower processing depth of the material surface. After nanosecond laser processing, a graphite layer is formed on the surface of CVD diamond, and surface grooves grooves and other damages appears when the laser power is higher, which can be suppressed by increasing the laser incident angle. The cracks on the surface of the material are attributed to the tensile stress in the graphite layer after cooling, and the size and number of cracks can be reduced by increasing laser incidence angle and decreasing laser power. Finally, the surface roughness (Sa) of CVD diamond dropped to be 1.3 μm by controlling the parameters, including laser incidence angle, laser power and laser scanning speed.